The present invention relates to processes for preparing tougheners for thermoset compositions, the toughener compositions formed thereby, toughened thermoset compositions containing the toughener compositions, and composite materials and articles formed from the toughened thermoset compositions.
Advanced composite materials having high heat resistance are of utility in high-performance structural applications in the construction, electronic, automotive, computer, aerospace, and electrical industries. Many of these advanced composite materials are based on the thermal curing (“thermosetting”) of liquid resin formulations which, upon crosslinking, form rigid, highly crosslinked polymeric matrices.
It is well known that such rigid, highly crosslinked polymeric matrices are brittle and have poor impact strength. Various toughening agents for these thermosetting materials have been developed, including particulate rubbers. The particulate rubbers can be derived from liquid rubbers (“LRs”) that have low viscosities and that tend to be miscible with the uncured liquid resin formulations. The LRs can be mixed with the uncured liquid resin formulations, then typically phase separate upon curing (crosslinking) of the thermoset resins to form rubbery microdomains in the crosslinked polymeric matrix of the thermosetting resin. These rubbery microdomains, typically having a size of about 0.1 to about 5 micrometers, help to toughen the rigid crosslinked polymeric matrix while maintaining heat resistance and dimensional stability of the matrix. Various types of LR tougheners are disclosed, for example, in Mulhaupt, R., “Flexibility or Toughness?—The Design of Thermoset Toughening Agents”, Chimia 44 (1990), pp. 43-52.
Most, if not all, known LR tougheners contain functional groups. It is generally believed that these functional groups enhance the interfacial adhesion of the phase-separated rubbery domain to the crosslinked polymeric matrix by allowing covalent chemical bonding between the functional groups of the LRs and the functional groups of the crosslinkable polymer resin. Often the functional groups of the LRs are located at the ends of polymer chains, denoted “terminally functional” or “functionally terminated” LRs. In addition, terminal functional groups tend to increase the molecular weight of the polymer chains in the rubbery microdomains during curing, which also tends to improve impact strength.
An important design parameter for an LR toughener is its molecular weight. While phase separation and toughness typically improve with increasing molecular weight of the LR, compatibility between the LR and the uncured liquid thermoset resins typically improves with decreasing molecular weight. Ideally, the LR is miscible with the uncured liquid thermoset resin because single-phase liquid thermoset resin formulations have lower viscosities and better processing characteristics than multi-phase liquid thermoset resin formulations, which tend to exhibit complex rheological behavior.
Commercially available LR tougheners include functionally terminated copolymers of butadiene and acrylonitrile, and include carboxy-terminated copolymers (known as “CTBN”), amino-terminated copolymers (“ATBN”), vinyl-terminated copolymers (“VTBN”), and epoxy-terminated copolymers (“ETBN”). Of the two common thermosetting resins, epoxy and unsaturated polyester, the epoxy resins have proved to be amenable to toughening by low levels of CTBN or ATBN copolymers. The carboxylic acid and amine functional groups are known to enhance the miscibility of the LR tougheners in uncured epoxy resins. These liquid rubbers are also effective in improving crack resistance and impact strength, while minimally affecting the heat distortion properties of the normally brittle epoxy resins.
Unfortunately, there are several drawbacks associated with terminally functional LRs. One is that the functional groups tend to react and crosslink, thereby increasing the molecular weight of the LR. This can lead to viscosity increases and/or reduced miscibility of the LR/thermoset resin liquid blend, which makes processing difficult. This problem is particularly severe for polymers that have reactive functional groups at each end of the polymer chain. Another drawback is that strong interactions and/or reaction between terminal functional groups and reactive groups on the thermosetting resins also causes increased viscosity and reducing miscibility (phase separation) of the LR/liquid thermoset blend.
Another drawback is that while CTBN and ATBN LRs are widely used with epoxy liquid thermoset resins, the incorporation of low levels of CTBN and/or ATBN LRs into unsaturated polyester resins results in negligible improvement in crack resistance and impact strength at the expense of reducing the heat distortion characteristics of the cured resin matrix.
The aforementioned problems thereby preclude the use of such blends, especially those based on unsaturated polyester thermoset resins, in processing operations that require low viscosities, such as pultrusion, resin transfer molding, and spray-up. Moreover, when preparing LR/thermoset liquid resin blends, the end-user must carefully measure and mix these individual components. This hinders the preparation of “one-pack” LR/thermoset liquid resin blends.
The inventors hereof have previously found that liquid rubbers that are ordinarily immiscible in liquid thermoset resins can be made miscible by the addition of at least one non-functional aromatic end-group to the polymer chains of such liquid rubber compositions. These new liquid rubber compositions can be controlled to phase separate into rubbery microdomains upon curing of thermoset resins, including unsaturated polyester thermoset resins. The resulting new composite materials are found to improve the fracture toughness of cured thermoset resins while maintaining dimensional and heat resistance. The new LRs are highly advantageous as the LR/thermoset liquid resin blends remain miscible in the uncured state over time, have low viscosity and easy processability, and are chemically stable.
The new LRs having at least one non-functional aromatic end group are preferably manufactured using hydrocarbon free radical initiators in aromatic solvents, preferably in a continuous flow stirred tank reactor (“CFSTR”). Continuous flow reactors are often preferred for manufacturing processes because they can often improve production capacity. In continuous reactors the reactants are added and the products are removed simultaneously. In semi-batch reactors the reactants are added continuously while the reaction proceeds, and then the products are removed. In batch reactors reactants are added, the reaction proceeds, and then the products are removed.
While the above process is suitable for its intended purposes, there nonetheless remains a continuing need for improvements in the manufacture of such LRs, in particular methods that allow for manufacture by either contininous, semi-batch, or batch processes. It would be a further advantage if the initiator were commonly available, while still providing control of the molecular weight of the liquid rubbers, thus allowing tailoring of the properties of the LR/thermoset compositions, including stability and processability of the LR/thermoset liquid resins, as well as ductility, crack resistance, and impact strength of LR/thermoset composites. It has been unexpectedly found by the inventors hereof that nonfunctional aromatic-terminated liquid rubbers can be formed using nonaromatic or monoaromatic peroxide initiators by continuous, semi-batch, or batch processes.